OpenCV Error in cvtColor function: assertion failed (scn == 3 || scn == 4)

I just want to upload a video file, convert it to grayscale and display it. Here is my code:

import numpy as np
import cv2

cap = cv2.VideoCapture('cars.mp4')

while(cap.isOpened()):
    # Capture frame-by-frame
    ret, frame = cap.read()
    #print frame.shape   


    # Our operations on the frame come here
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

    # Display the resulting frame
    cv2.imshow('frame',gray)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break


# When everything done, release the capture
cap.release()
cv2.destroyAllWindows()

      

        
        
        
      

    

The video plays in grayscale until the end. Then it freezes and the window becomes unresponsive and I get the following error in the terminal:

OpenCV Error: Assertion failed (scn == 3 || scn == 4) in cvtColor, file /home/clive/Downloads/OpenCV/opencv-2.4.9/modules/imgproc/src/color.cpp, line 3737
Traceback (most recent call last):
  File "cap.py", line 13, in <module>
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
cv2.error: /home/clive/Downloads/OpenCV/opencv-2.4.9/modules/imgproc/src/color.cpp:3737: error: (-215) scn == 3 || scn == 4 in function cvtColor

      

        
        
        
      

    

I have uncommented the print frame.shape statement. It stores print 720,1028,3. But after the video is played to the end it freezes and after a while it closes and returns

print frame.shape   
AttributeError: 'NoneType' object has no attribute 'shape'

      

        
        
        
      

    

I understand that this massage of assertion failure usually means that I am trying to transform an empty image. I added a check for an empty image before starting to process it with the if (ret): statement. (Any other way to do this?)

import numpy as np
import cv2

cap = cv2.VideoCapture('cars.mp4')

while(cap.isOpened()):
    # Capture frame-by-frame
    ret, frame = cap.read()
    #print frame.shape   

    if(ret): #if cam read is successfull
        # Our operations on the frame come here
        gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

        # Display the resulting frame
        cv2.imshow('frame',gray)
        if cv2.waitKey(1) & 0xFF == ord('q'):
            break
    else:
        break

# When everything done, release the capture
cap.release()
cv2.destroyAllWindows()

      

        
        
        
      

    

This time, the video plays to the end, and the window still freezes and closes after a few seconds. This time I am not wrong. But why window freeze? How can I fix this?